The invention relates to the scanning of cinematographic film to produce electrical signals corresponding to the images stored on the film. In particular, the invention relates to telecine; a type of film scanner which converts film images to television signals.
Telecine or film scanning equipment used to produce such signals from cinematographic film have been known for many years, and are described for example in “TV and Video Engineers Reference Book” Chapter 39 Butterworth and Heinemann ISBN 0-7506-1021-2. There are two main types of film scanner: diffuse light illumination, and spot scanning. The former uses a diffuse light source to illuminate a frame of film and optics to image the illuminated frame on to a light detector, such as a line array or area array CCD detector. Spot scanning systems use a flying spot of light to successively illuminate points of film. Light transmitted by the film is collected by light collection optics and converted to electrical signals by a detector. The invention relates particularly to the type of telecine or film scanner that uses a scanning light source such as the known Ursa™ or C-Reality™ telecines of Cintel International Limited.
The illumination and light collection systems of the C-Reality™ telecine is described in simple terms with reference to FIG. 1, which shows a schematic form the main functional components. However, it should be noted that the design detail has been simplified for ease of description as would be known by the skilled person.
A Cathode Ray Tube (CRT) 1 produces a raster scan that is imaged onto the film 3 by an imaging lens group 2 light passing through the film is modulated by the colour and density of the film at each location or pixel scanned, this light being subsequently analysed into it's red, green, and blue components. The lenses 4 and 6 collect the light from the film and apply it to the blue Avalanche Photo Diode (APD) sensor 7 via the blue separating dichroic mirror 5. The mirror 5 transmits blue light but reflects red and green to the red separating dichroic mirror 8, which reflects the red light through lens 9 to the red APD sensor 10. The remaining green light passes through the mirror 8 and lens to the green APD sensor 12. The three electrical colour signals are then passed through electronic processing circuits, converted into a television signal format and provided as output signals which are typically then recorded on video tape equipment. Alternatively, the signals may be converted to video data format and stored in data recording equipment.
We have appreciated that it would be beneficial to collect more of the light that is scattered by scratches on the surface of, or other deformities in or on, the film. Such scratches cause scattering of the light, which may then be lost from the optical system and cause a reduction in the signal received by the photo sensors, if this light could be effectively collected then the visibility of the scratches in the resultant electrical images would be much reduced. FIG. 1 shows the path of the imaged rays passing through the film, and also shows a ray typical of that scattered by a film scratch, it can be seen that this latter ray misses the collecting lenses 6,9,11.
Various systems for reducing the visibility of scratches have been proposed. International patent WO 83/02869 A1 and U.S. Pat. No. 4,481,414 describe various methods of improving the collection of such scattered light by imaging and or by reflective means. UK patent GB 1409153 suggests a scheme of using additional photo sensors to collect some of this scattered light to enable the substitution of a suitable alternative signal. UK patent application GB 2323495A also discloses improved collection of the scattered light by imaging and reflective means and the use of additional sensors to collect the scattered light and add this in suitable proportion to the main signal.
One problem we have appreciated, however, is that there is a limit to the amount of this scattered light that can be collected in a practical optical imaging system. This limit is due to the physical limitations of an optical imaging system described by the LaGrange optical invariant that can be referenced at page 2 to 8 of the “Handbook of Optics” published by the McGraw-Hill Book Company ISBN 007-047710-8.
In short, in any optical system comprising only lenses, the product of the image size and ray angle is a constant, or invariant, of the system. This can be related to the numerical aperture which is the ratio of the physical aperture of a lens divided by the focal length, which for the maximum image is the ray angle. Thus, in a raster scan telecine described above, the optical invariant shows that the product of the maximum scanned film dimension and the numerical aperture of the rays passing through the film will, in a theoretically perfect optical system, be equal to the product of the maximum active sensor dimension and the numerical aperture of the rays arriving at the sensor. The dimensions of the film is a fixed requirement and that of the sensor is chosen to give best signal performance, whilst the numerical aperture of the rays arriving at the sensor are limited by the sensor characteristics. The maximum numerical aperture that can be collected is therefore limited by these parameters.
Light scattered by a film scratch will normally include a significant proportion outside of this angle that cannot be collected. Any attempt to increase the angle of scattered light collected will result in a loss of imaged light collected or a loss of light from the extremes of the image; neither effect can be tolerated in high performance equipment.
A known solution is to use a large sensor, however in the example of the C-Reality telecine the sensor is for reasons of best performance chosen to be an avalanche photo diode of 10 mm diameter. It is also known that the effective numerical aperture of the photo sensor can, in some instances, be increased by the use of a high refractive index substance fitted between the active surface of the photo sensor and the optical system.
To try and overcome these limitations of an imaging system, a further known method of improving the scattered ray collection is to use an integrating sphere or cylinder to collect all light from film (both scratched and unscratched areas). This method will collect substantially all angles of light rays from the film, but suffers the disadvantage of being very inefficient so would not collect enough intensity of the light from the film (from both scratched and unscratched regions). As a result, there is a loss of quality.